🚀 go-pugleaf

> |> You must be thinking of 1st Party DMA, not bus mastering.
> 
> What is "1st Party DMA"?
 
 
Ok, ok, I've been getting a lot of mail lately asking me 
this.  So I quote without permission from Roedy Green's 
article in the September 92 computer paper:
 
"DMA (Direct Memory Access) is a method of having a device 
such as the disk controller work independently of the CPU, 
read the disk and directly deposit the data in RAM, all 
without CPU intervention.  There are three types of DMA; third 
party DMA where the motherboard DMA chip controls the 
operation, first-party DMA where the disk controller itself 
takes over that function, and bus mastering where the disk
controller commandeers the entire bus for short bursts to blast 
the data into RAM.
 
On the ISA bus, third-party DMA is too slow to use at all.  
The best you can hope for is first-party DMA--used only on 
high-end SCSI host adapters.  In contrast, the EISA cards use 
the extremely fast bus mastering method.  Often ISA 
first-party DMA is erroneously billed as bus mastering."
 
blah blah
 
"First-party DMA SCSI host adapters are able to work 
independently, allowing the CPU to work on other tasks.  Under 
DOS the CPU just spins its wheels waiting for the disk to 
finish, but under OS/2, the CPU does other useful work.  This 
independence is what makes SCSI so fast under OS/2"
 
I believe I might have been wrong in saying that VESA's VL-BUS 
spec doesn's support first party DMA.  What I meant to say is 
that it doesn't support multi-plexing.  However the mezzanine 
extension of the spec will have multi-plexing, 64-bit 
processor support, up to 10 local bus devices along with other 
goodies.  It's current problem being only local bus device can
the local bus path at one time.
 
Later.